The invention relates to an image sensor for reading images, and more particularly to an image sensor and a method of driving such image sensor in which a plurality of light-receiving element arrays are arranged on a single substrate and the respective light-receiving element arrays have filters for different colors (e.g., red, green and blue) so that color images can be read.
A conventional color image sensor in which a plurality of light-receiving element arrays are arranged on a single substrate is constructed as shown by plan and sectional views in FIGS. 6 and 7, respectively.
On a substrate 60 are four vertically (an auxiliary scanning direction Y) juxtaposed arrays of pixel electrodes 61a, 61b, 61c and 61d, each array extending in a main scanning direction X, with corresponding lead electrodes 62a, 62b, 62c and 62d drawn out therefrom. The lead electrodes 62a and 62d extending from the two externally arranged pixel electrodes 61a and 61d are drawn out in directions opposite to each other, while the lead electrodes 62b and 62c extending from the two internally arranged pixel electrodes 61b and 61c are drawn out in directions opposite to each other passing between the pixel electrodes 61a and between the pixel electrodes 61d, respectively. An amorphous semiconductor film 63 is formed on the pixel electrodes 61, and a common transparent electrode 64 is formed on the amorphous semiconductor film 63. The portions interposing the amorphous semiconductor film 63 between the pixel electrodes 61 and the common transparent electrode 64 form photosensitive pixel regions, or light-receiving elements. On the common transparent electrode 64, color filters 66a, 66b and 66c, respectively confronting the pixel electrodes 61a, 61b and 61c, are disposed to color separate image data. The color filters 66a, 66b and 66c are red, green and blue ones respectively.
Ends of the lead electrodes 62a through 62d are connected to IC chips (not shown) through wire bonding. Each IC chip forms a drive circuit for extracting electric charges stored at each light-receiving element array. A simple equivalent circuit of a light-receiving element array and its drive circuit is shown in FIG. 8. In FIG. 8, reference numerals 71a, 71b, 71c and 71d respectively correspond to the pixel regions shown in FIG. 6 (hereinafter referred to as "R (red) pixel region, G (green) pixel region, B (blue) pixel region, and W (luminance) pixel region") and are connected to common output lines 73a, 73b, 73c and 73d through signal read switches 72a, 72b, 72c and 72d, respectively. The common output lines 73a, 73b, 73c and 73d are connected to A/D (analog-to-digital) converters 74a, 74b, 74c and 74d, respectively. The output side of the A/D converter 74b is connected to an n-stage delay register 75; the output side of the A/D converter 74c is connected to a 2n-stage delay register 76; and the output side of the A/D converter 74d is connected to a 3n-stage delay register 77 (where n is the number of pixels in a pixel region for each color extending in the main scanning direction).
A document placed over the light-receiving element arrays are movable in the auxiliary direction by document feed means such as rollers. Let it be assumed that an image on the document to be read is represented by pixels P11-P1n, P21-P2n, P31-P3n, . . . as shown in FIG. 9. When the pixels P11-P1n come to a position corresponding to the W pixel region 71d, the switch 72d is selectively and sequentially turned on from left, thereby sequentially producing the luminance data of the pixels P11-P1n as electric signals at the common output line 73d. The respective electric signals are then converted into digital values by the A/D converter 74d, transferred to the delay register 77, and stored at the 1st to n-th stages of the delay register.
Then, when the pixels P11-P1n move to a position corresponding to the B pixel region 71c, the switch 72c is selectively and sequentially turned on from left, thereby sequentially producing the blue color data of the pixels P11-P1n as electric signals at the common output line 73c. The respective electric signals are then converted into digital values by the A/D converter 74c, transferred to the delay register 76, and stored at the 1st to n-th stages of the delay register 76. Similarly, the green color data of the pixels P11-P1n are read from the G pixel region 71b and stored at the 1st to n-th stages of the delay register 75. Further, the red color data of the pixels P11-P1n are read from the R pixel region 71a. Since the delay registers 75, 76 and 77 transfer the data based on a predetermined clock, when the red signals are sequentially outputted from the pixels P11-P1n from the A/D converter 74a, the green, blue and luminance signals of the same pixels are outputted from the delay registers 75, 76 and 77 in synchronism. Data of the other pixels P21-P2n, P31-P3n, . . . are read similarly (see Japanese Patent Unexamined Publication No. Sho. 60-113573).
However, such conventional construction as described above requires a separate drive circuit to be provided at each line to extract the electric signals from each light-receiving element array, thereby making the image sensor expensive.
Further, the line capacitances of the lead electrodes 62b and 62c drawn out from the pixel electrodes 61b and 61c constituting the two internal light-receiving element arrays are not equal to those of the lead electrodes 62a and 62d drawn out from the pixel electrodes 61a and 61d constituting the two external light-receiving element arrays due to different lengths of their lines. Therefore, even if light is projected equally to the respective light-receiving element arrays and the electric charges generated at the respective light-receiving element arrays are equal (Q=CV), the output voltages (V) of the respective light-receiving element arrays are not equal because their line capacitances (C) are not equal.
Furthermore, the lead electrodes 62a, 62b and 62c of the pixel electrodes 61a, 61b and 61c constituting the respective light-receiving element arrays are wire-bonded to the corresponding drive IC circuits, and this disadvantageously increases the number of wires used for the wire-bonding impairing reliability.